can anyone tell me if Jessie's (LMDE 2) Firefox ESR 52.9 will ever update to 60.x ESR? For example, I read about the package "RUST" that is needed but not right included in Jessie (or something like that), but no concrete answers as to whether Jessie can still get the next higher FF ESR version.

Btw. it's already long overdue... and I feel a little bit insecure now.

You must have installed firefox-esr manually. As you've investigated, Debian jessie only has firefox-esr 52 package and LMDE 2 only has firefox 60 package (though it is also the ESR). If you want the newer Firefox, install the firefox package from LMDE 2.

Why Debian jessie is stuck with Firefox 52 I don't know. Debian stretch has Firefox 60 just like LMDE 2 and 3.

Thanks for your effort and time. From now on and until the end of the year I am right on the LMDE FF ESR repository path.

btw.
As far as I remember, I've chosen the Debian FF ESR branch since the beginning of LMDE 2, because the initial LMDE2 repostiory had only inserted the normal firefox (without ESR support) in it.
If I am wrong I have at least now taken the right path.